Composite structures typically include continuous reinforcing fibers embedded in a resin matrix. A composite laminate is a type of composite structure comprising a layup of composite plies arranged in a stack. The individual composite plies of a composite layup may be pre-impregnated with resin (e.g., prepreg plies) prior to stacking. The stack of prepreg plies may be arranged such that the continuous reinforcing fibers in each ply are oriented in a specific direction. Heat may be applied to the stack to reduce the viscosity of the resin in each ply to allow the resin to intermingle with the resin of adjacent plies while the stack is consolidated under pressure to remove voids and volatiles from within the composite layup. The resin may be cured or solidified into a hardened state and passively or actively cooled resulting in a composite structure. Alternatively, instead of using prepreg plies, the composite plies may be provided as dry fiber preforms arranged in a stack. Liquid resin may be infused into the stack while heat and/or pressure are applied to consolidate and cure the resin after which the layup may be passively or actively cooled to result in a composite structure.
The ability to tailor the direction of the reinforcing fibers in each ply of a composite layup results in a composite structure with significant performance advantages. Such performance advantages include a high specific strength and high specific modulus of elasticity relative to the specific strength and modulus of metallic structures. Unfortunately, conventional composite laminates possess several characteristics that may detract from their performance advantages. For example, conventional composite laminates may be susceptible to separation at the resin-fiber interface due to the absence of crack-arresting features within the composite laminate. In addition, a conventional composite assembly may have relatively low mode II interlaminar shear strength or peel strength at the interface between co-bonded or co-cured composite laminates that make up the composite assembly.
A conventional composite laminate may also possess relatively low electrical conductivity which may present challenges in transporting and distributing electrical current through a composite structure such as in the event of a lightning strike. In addition, composite laminates that interface with metallic components may be susceptible to corrosion as a result of oxidation or reduction reactions that may occur between the composite laminate and metallic material. Furthermore, conventional dry fiber composite plies may lack sufficient tack to enable the dry fiber plies to stick together to allow for controlled stacking of the dry fiber plies into a preform.
Attempts to resolve the issue of separation at the resin-fiber interface of conventional composite laminates include randomly distributing thermoplastic material in bulk throughout a composite layup. Although the random distribution of thermoplastic material may improve the mode II interlaminar strength, the lack of control at the resin-fiber interface in conventional composite laminates results in low mode I interlaminar strength which may present challenges in preventing crack propagation within fiber tows. Attempts to address low mode II interlaminar shear strength at the interface between composite laminates of a conventional composite assembly include the addition of tougheners in the resin. Unfortunately, resin tougheners may have a relatively high molecular weight that may undesirably increase the viscosity of the resin which may inhibit resin flow during infusion of fiber preforms. Attempts to address the issue of low electrical conductivity in conventional composite laminates include the addition of metallic meshes or foils across the surface of composite plies. Unfortunately, the addition of separate metallic meshes or foils increases the cost, complexity, and production time of a composite structure.
Attempts to prevent corrosion at the interface between a composite laminate and a metallic part include adding a separate layer of fiberglass at the interface to act as a barrier ply against corrosion. Unfortunately, the addition of fiberglass increases the cost and complexity of manufacturing a composite laminate. The problem of low tack in conventional dry fiber composite plies has been addressed by adding epoxy binders or nylons in the resin, or by using soldering irons to locally heat and tack composite plies together. Unfortunately, epoxy binders or nylons have finite properties that limit the range of temperatures and pressures required to form a ply stack of dry fiber preforms. The local tacking together of composite plies using soldering irons is a time-consuming process that adds to the production time of a composite structure.
As can be seen, there exists a need in the art for a composite laminate and manufacturing method that provides performance improvements such as improved crack-resistance, improved interlaminar shear strength, increased electrical conductivity and corrosion resistance, and improved tack in a broad range of temperatures.